The present invention relates to a radial anisotropic sintered magnet and a method of producing a radial anisotropic sintered magnet. The present invention also relates to a cylindrical magnet rotor for a synchronous permanent magnet motor such as a servo-motor or a spindle motor, and an improved permanent magnet type motor using the cylindrical magnet rotor.
Anisotropic magnets, each produced by pulverizing a material having magnetic anisotropic crystals, such as ferrite or a rare earth alloy, and pressing the pulverized material in a specific magnetic field, have been extensively used for loudspeakers, motors, measuring instruments, and other electric components. Of these anisotropic magnets, those having radial anisotropy have been advantageously used for AC servo-motors, DC brushless motors, and the like because of excellent magnetic characteristics, free magnetization, and no need of reinforcement for fixing the magnets unlike segment type magnets. In particular, along with the recent tendency toward higher performances of motors, it has been required to develop long-sized radial anisotropic magnets.
Magnets oriented in radial directions have been produced by a vertical-field vertical molding process or a backward extrusion molding process. According to the vertical-field vertical molding process, magnetic fields are applied toward the center of a core in opposed directions parallel to the pressing direction, that is, the vertical direction. The magnetic fields are impinged against each other at the center of the core, to be turned in radial directions, whereby a magnet powder is oriented in the radial directions. To be more specific, as shown in FIGS. 2A and 2B, a vertical-field vertical molding process is carried out by packing a magnet powder 8 in a cavity between a die 3 and a core composed of an upper core part 4 and a lower core part 5, applying magnetic fields, generated by upper and lower orientation magnetic field coils 2, toward the center of the core in opposed directions parallel to the pressing direction, and pressing the packed magnet powder 8 in the vertical direction. In this process, the magnetic fields applied in the opposed directions parallel in the vertical direction are impinged against each other at the center of the core to be turned in radial directions, to pass through the die 3 toward a molding machine base 1, and the packed magnet powder 8 is pressed in the magnetic fields circulating in this magnetic circuit, to be thereby oriented in the radial directions. In the figures, reference numeral 6 denotes an upper punch and reference numeral 7 denotes a lower punch.
In this way, in the vertical-field vertical molding process, the magnetic fields generated by the coils form a magnetic path of the core, the die, the molding machine base, and the core. In this case, to reduce the leakage of the magnetic fields, a ferromagnetic material, particularly, a ferrous material is used as a material forming the magnetic path. A magnetic field intensity for orienting a magnet powder is, however, determined as follows. It is assumed that a core diameter be B (inner diameter of the packed magnet powder), a die diameter be A (outer diameter of the packed magnet powder), and a height of the packed magnet powder be L. The magnetic fluxes having entered the core composed of the upper and lower core parts are impinged against each other at the center of the core, to be turned in radial directions, and pass through the die. The amount of the magnetic fluxes having passed the core is determined by a saturated magnetic flux density of the core. The magnetic flux density of the core, if made from iron, is about 20 kG. Accordingly, the orientation magnetic field at each of the inner diameter and the outer diameter of the packed magnet powder is obtained by diving the amount of the magnetic fluxes having passed through the core by each of an inner area and an outer area of the packed magnet powder, as expressed below.2·π·(B/2)2·20/(π·B·L)=10·B/L (inner periphery)2·π·(B/2)2·20/(π·A·L)=10·B2/(A·L) (outer periphery)
The magnetic field at the outer periphery is smaller than that at the inner periphery. Accordingly, to obtain desirable orientation in the whole packed magnet powder, the magnetic field at the outer periphery, which is expressed by the equation of 10·B2/(A·L), is required to be 10 kOe or more. As a result, by setting the magnetic field at the outer periphery to 10 (that is, 10·B2/(A·L)=10), an equation of L=B2/A is given. By the way, since the height of a molded body is about half the height of a packed magnet powder and is further reduced to about 0.8 by sintering, the height of a finished magnet becomes very smaller than the height of the packed magnet powder. In this way, the size, that is, the height of a magnet allowed to be oriented is determined by the shape of a core because the magnetic saturation of the core determines the intensity of the orientation magnetic field. This is the reason why it has been difficult to produce cylindrical anisotropic magnets longer in the axial direction, particularly, when the magnets have small diameters.
On the other hand, the backward extrusion molding process requires a large, complicated molding machine, to degrade the production yield. Accordingly, it has been difficult to produce-radial anisotropic magnets at a low cost.
In this way, it has been difficult to produce radial anisotropic magnets in any method, and has been further difficult to produce radial anisotropic magnets on the large scale at a low cost, resulting in the significantly raised cost of motors using the radial anisotropic magnets thus produced.
In the case of producing radial anisotropic ring-shaped magnets by using a sintering process, there arises the following problem: namely, if a stress generated in the steps of sintering and cooling for aging due to a difference between a coefficient of linear thermal expansion in the C-axis direction of the magnet and a coefficient of linear thermal expansion in the direction perpendicular to the C-axis direction of the magnet is larger than a mechanical strength of the magnet, there may occur cracks. For example, in the case of producing R—Fe—B based sintered magnets, as disclosed in Hitachi Metals Technical Report Vol. 6, p33–36, only a magnet shaped with a ratio between an inner diameter and an outer diameter set in a range of 0.6 or more has been produceable without occurrence of cracks. Further, in the case of producing R—(Fe—Co)—B based sintered magnets, since Co replaced from Fe is not only contained in a 2-14-1 phase as a main phase in an alloy structure but also forms R3Co in an R-rich phase, a mechanical strength is significantly reduced, and since the Curie temperature is high, a difference between a coefficient of linear thermal expansion in the C-axis direction and a coefficient of linear thermal expansion in the direction perpendicular to the C-axis direction in a temperature range from the Curie temperature to room temperature at the time of cooling becomes large, with a result that a residual stress as a cause of cracking becomes large. For this reason, the shape limitation to the R—(Fe—Co)—B based radial anisotropic ring-shaped magnets is more strict than the shape limitation to the R—Fe—B based magnets not containing Co. In actual, only the R—(Fe—Co)—B based magnets shaped with a ratio between an inner diameter and an outer diameter set in a range of 0.9 or more have been stably produceable. For the same reason, ferrite magnets and Sm—Co based magnets have been difficult to be stably produced without occurrence of cracks.
From the result of examination by F. Kools on a ferrite magnet (F. Kools: Science of Ceramics. Vol. 7, (1973), 29–45), a residual stress in a peripheral direction, regarded as a cause of cracks of radial anisotropic magnets in the step of sintering and cooling for aging, is expressed by the following equation:σθ=ΔTΔαEK2/(1−K2)·(Kβkηk−1−Kβ−kη−k−1−1)  (1)where                σ74: stress in peripheral direction        ΔT: difference in temperature        Δα: difference in coefficient of linear thermal expansion (α∥−α⊥)        E: Young's modulus in orientation direction        K2: anisotropic ratio of Young's modulus (E⊥/E∥)        η: position (r/outer diameter)        βk: (1−ρ1+k)/(1−ρ2k)        ρ: ratio between inner diameter and outer diameter (inner diameter/outer diameter)        
In the equation (1), the term exerting the largest effect on a cause of cracking is Δα: difference in coefficient of linear thermal expansion (α∥−α⊥). For ferrite magnets, Sm—Co based rare earth magnets, and Nd—Fe—B based rare earth magnets, a difference between a coefficient of thermal expansion in the crystal direction and a coefficient of thermal expansion in the direction perpendicular to the crystal direction (anisotropy in thermal expansion) appears at the Curie temperature and increases with a decrease in temperature at the time of cooling, with a result that a residual stress becomes larger than the mechanical strength, resulting in occurrence of cracks.
The stress due to a difference between the thermal expansion in each orientation direction of a cylindrical magnet and the thermal expansion in the direction perpendicular to the orientation direction of the cylindrical magnet, expressed in the above-described equation (1), is generated due to the fact that the cylindrical magnet is radially oriented along the radial direction. Accordingly, if a cylindrical magnet containing a suitable volume % of a portion oriented in directions different from radial directions is produced, such a cylindrical magnet will be probably not cracked. For example, a cylindrical magnet oriented in one direction perpendicular to the axial direction of the cylindrical magnet, which is produced by a horizontal-field vertical molding process, is not cracked even if the cylindrical magnet is either of a ferrite magnet, an Sm—Co based rare earth magnet, an Nd—Fe(Co)—B based rare earth magnet.
Even in the case of using a cylindrical magnet of a type different from a radial anisotropic magnet, if the cylindrical magnet can be subjected to multipolar magnetization so as to obtain a sufficiently high magnetic flux density and a small variation in magnetic fluxes between magnetic poles, such a cylindrical magnet can be used as a magnet for high-performance permanent magnet motors. For example, a method of producing a cylindrical multipolar magnet for permanent magnet motors different from any radial anisotropic magnet has been proposed in the paper “Electricity Society Magnetics Research Group, Material No. MAG-85-120 (1985)”. In this method, a cylindrical multipolar magnet is produced by preparing a cylindrical magnet oriented in one direction perpendicular to the axial direction of the cylindrical magnet by a horizontal-field vertical molding process and subjecting the cylindrical magnet to multipolar magnetization. The magnet oriented in one direction perpendicular to the axial direction of the cylindrical magnet (hereinafter, referred to as “diametrically oriented cylindrical magnet”) produced by the horizontal-field vertical molding process is advantageous in that the height of the magnet can be made as large as possible (about 50 mm or more) within the allowable range of a cavity of a pressing machine and further a number of the molded bodies can be formed by one pressing (hereinafter, referred to as “multiple pressing”), with a result that inexpensive cylindrical multipolar magnets for permanent magnet motors can be provided in place of expensive radial anisotropic magnets.
The above-described cylindrical magnet, produced by preparing a diametrically oriented cylindrical magnet by the horizontal-field vertical molding process and subjecting the cylindrical magnet to multipolar magnetization, however, has a problem from the practical viewpoint. Namely, a magnetic pole located near in the orientation magnetic field direction has a high magnetic flux density but a magnetic pole located in a direction perpendicular to the orientation magnetic field direction has a low magnetic flux density, and accordingly, when a motor incorporated with the magnet is rotated, there may occur an uneven torque due to a variation in magnetic flux density between the magnetic poles. In this way, such a cylindrical magnet cannot be regarded as usable from the practical viewpoint.
To solve the above-described problem, a patent document 1 has proposed a technique in which, assuming that the number of magnetized poles in the peripheral direction of a cylindrical magnet produced by the horizontal-field vertical molding process so as to be oriented in one direction perpendicular to the axial direction of the cylindrical magnet is 2n (n: positive integer larger than 1 and smaller than 50), the number of teeth of a stator to be combined with the cylindrical magnet is set to 3 m (m: positive integer larger than 1 and smaller than 33). A patent document 2 has proposed a technique in which, assuming that the number of magnetized poles in the peripheral direction of a cylindrical magnet produced by the horizontal-field vertical molding process so as to be oriented in one direction perpendicular to the axial direction of the cylindrical magnet is k (k: positive even number larger than 4), the number of teeth of a stator to be combined with the cylindrical magnet is set to 3k·j/2 (j: positive integer larger than 1). A patent document 3 has proposed a technique in which an uneven torque of a cylindrical magnet oriented in one direction perpendicular to the axial direction of the cylindrical magnet is reduced by dividing the cylindrical magnet into a plurality of cylindrical magnet units, and stacking the cylindrical magnet units to each other in such a manner that the cylindrical magnet units are sequentially offset from each other at a specific angle in the peripheral direction.
In each of the techniques disclosed in the patent documents 1 to 3, although the uneven torque can be reduced, the volume ratio of a diametrically oriented portion to the total volume of the ring-shaped magnet is small, with a result that a total torque of a motor incorporated with the magnet is as small as 70% of a total torque of a motor incorporated with a radial anisotropic magnet having the same magnetic characteristics. Accordingly, the magnet disclosed in each of the patent documents 1 to 3 has been not practically used.
The documents used for above description are as follows:                Patent Document 1: Japanese Patent Laid-open No. 2000-116089        Patent Document 2: Japanese Patent Laid-open No. 2000-116090        Patent Document 3: Japanese Patent Laid-open No. 2000-175387        Non-patent Document 1: Hitachi Metals Technical Report Vol. 6, p33–36        Non-patent Document 2: F. Kools: Science of Ceramics. Vol. 7, (1973), p29–45        Non-patent Document 3: Electricity Society Magnetics Research Group, Material No. MAG-85-120, 1985        